The desire to minimize the visibility of hearing aids in use has prompted improvements in technology used in their manufacture. For some time, it has been possible routinely to provide amplification efficiently packaged in concha or canal instruments.
It is also possible to build hearing aids which do not require a volume control to adjust loudness, because compression circuits can be used to adjust the output of the hearing aid amplifier below the uncomfortable loudness threshold of the user. Some amplifiers are able to amplify only quiet sounds, and can pass loud sounds (above a predetermined input level) through the instrument unamplified. These types of amplifiers avoid acoustic stress caused by over-amplification and eliminate the need for a volume control. Similar performance can be achieved with a well-adjusted linear amplifier which limits the maximum output sound pressure level below the discomfort level of the patient.
Therefore, once the hearing aid is placed in the ear, it no longer needs to be accessible to the user for frequent adjustments. The hearing aid can therefore be inserted quite deeply into the ear canal. Wearers of this kind of hearing aid benefit in various ways from a deep canal fitting.
One benefit is improved cosmetics. The deeper the hearing aid is seated in the canal, the less visible it becomes. At the extreme, it becomes invisible from outside the ear. However the hearing impaired person must have sufficient dexterity to insert and extract the instrument from the ear, and means must be provided as part of the hearing aid to make these operations routinely possible.
A second benefit is increased gain and power. The further the instrument is seated down the ear canal, the smaller is the residual volume between the sound outlet of the instrument and the eardrum. The smaller this volume becomes, the greater is the sound pressure level produced by the instrument. Therefore, for a given real ear output and gain, less output power is needed, and thus a smaller output transducer can be used. Both cosmetics and power consumption benefit from this effect.
A third benefit realized from seating the top of the instrument further down the ear canal is that the pinna and outer section of the ear canal are no longer occluded by the hearing aid. The unoccluded pinna and outer section of the ear canal are therefore able to function in their normal fashion, to provide directionality and frequency cues which assist the user in localizing the source of sound.
While the advantages offered by a "completely in the canal" (CIC) hearing aid are significant, there are also certain problems. With other types of hearing aids the portion of the shell which protrudes into the concha area determines and controls the insertion depth of the hearing aid into the ear canal and therefore also controls the distance, and thus the residual volume, between the inner end of the instrument and the tympanic membrane. Thus, every time the user inserts the hearing aid, he or she will be able to achieve the same insertion depth and hence the same residual volume. Because the hearing aid's output transducer always delivers its sound into the same volume, the prescribed acoustic levels produced by the instrument are always the same.
The above mentioned consistency is not always the case with CIC hearing aids. These are dependent on the first and possibly the second bend in the ear canal to locate and retain the instrument in the canal. Canals with pronounced bends can usually perform this function adequately. Canals with weak bends, or no bends at all, will not provide stable, repeatable or consistent positioning of the instrument in the canal. As a result, the perceived output power of the instrument will vary widely, in many cases seriously affecting user satisfaction.
A further problem is that it has been normal in the past to provide a short length of nylon or fishing line to assist in removing the CIC instrument from the ear canal. Such piece of line is attached to the faceplate of the hearing aid and protrudes a short distance into the concha when the hearing aid is properly located in the ear canal. To remove the hearing aid, the line is grasped and the hearing aid is pulled out. However if the canal bends do not retain the instrument securely, the instrument may work its way down the canal sufficiently far that the line disappears into the canal and is no longer accessible. In addition users may not wish to have a piece of line protruding from their ears.